This invention relates to substrate optical waveguides, and more specifically, to a substrate optical waveguide having fibers pigtailed thereto, as well as to the method of pigtailing or attaching the fibers thereto.
More generally, the invention relates to the attaching of such optical fibers to optical waveguide substrates of the type relating to lithium niobate (LiNbO.sub.3) technology or alternatively, lithium tantalate (LiTaO.sub.3) technology, and devices employing said technology which are manufactured as integrated optical circuit components such as phase modulators.
As is well known from U.S. Pat. No. 4,439,265, operation of integrated optic devices relies in part on the fact that electromagnetic radiation, i.e., optical or infrared radiation can propegrate through and be contained by layers of transparent materials. Such materials include lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3). As disclosed in the referenced U.S. patent, the materials crystallize in a so-called trigonal crystal system which has a threefold symmetry axis, conventionally identified as the z-axis or direction. The basal plane, i.e. the plane normal to the Z-direction, contains the unique x and y directions, arranged at right angles to each other. For the sake of simplicity, since propagation is preferably the same along either the x or y axis, reference will be made to only the y axis, it being understood that the same applies to the x-axis.
As a rule, optical radiation entering a crystal divides into two rays, called the ordinary ray and the extraordinary ray. The rays have polarization vectors at right angles to each other and in general, have different phase velocities implying the existence of two refractive indices in such crystals, which are termed in the ordinary refractive index n.sub.0 the extraordinary index n.sub.e.
Typically, the waveguides employed in integrated optics are typically a channel waveguide, more typically a thin narrow region having somewhat higher refractive index than the surrounding medium, with typical transverse dimensions of one to several micrometers of the radiation. The last requirement translates into typical transverse dimensions of integrated optics channel waveguides of one to several micrometers. Such guiding structures are produced by lithographic techniques akin to those used in integrated circuit technology.
Typical of methods employed for producing a light waveguide in substrate materials, in addition to the above-referenced U.S. patent, are the methods also disclosed in U.S. Pat. Nos. 3,837,827 to Caruthers, et al. and 4,284,663 to Caruthers, et al., as well as the most preferred method as disclosed in U.S. application Ser. No. 908,066, filed Sept. 16, 1986, now abandoned, which is commonly assigned. It is these types of devices on which the method in accordance with the invention is practiced because these devices, be they a simple a light guide or an active device such as a phase modulator having electrodes thereon, are useless unless there is a means for transmitting light into and out of the devices.
Accordingly, it becomes necessary to align and attach optical fibers to the end of the waveguide defined in the substrate in a very precisely aligned manner to reduce losses in transmissions between the mediums.
U.S. Pat. No. 4,639,074 teaches a method of aligning a fiber waveguide to a waveguide substrate wherein one or more fibers are held in silicon v-grooves and mated in an overlap fashion with the waveguide substrate such that the plurality of degrees of freedom are automatically aligned. In order to attempt to minimize transmission losses, the silicon substrate is set to overlap the top surface of the waveguide substrate, and since the waveguide defined in the waveguide substrate, is generated by in-diffusion thereinto, precise alignment between fiber and light guiding region is not always possible.
A further disadvantage with this arrangement is that it requires an active alignment along one axis before the fiber array is secured.